Unable to lock the administration directory (/var/lib/dpkg/) is another process using it?

This should be used as last resort. If you use this carelessly you can end up with a broken system. Please try the other answers first before doing this.

You can delete the lock file with the following command:

sudo rm /var/lib/apt/lists/lock

You may also need to delete the lock file in the cache directory

sudo rm /var/cache/apt/archives/lock
sudo rm /var/lib/dpkg/lock

After that, try opening Synaptic again.

(Note: my original answer was extensively edited by Guillem Jover, the primary dpkg developer.)

I see pretty much all the answers recommend deleting the lock. That must never be done, it is always preferable to kill dpkg (which is supposed to be resilient against that kind of event), than to even think about removing its lock file (where its presence does not indicate the lock being held). The locks are acquired when a dpkg or an apt process is running, and are released (by the kernel if necessary) when the processes complete or are killed. Newer dpkg and apt versions will print the PID of the process holding the contended lock file, and apt now even waits by default for the locks to be released. This is covered in the dpkg FAQ.

If you try:

sudo fuser -vik -TERM /var/lib/dpkg/lock /var/lib/dpkg/lock-frontend /var/lib/apt/lists/lock
sudo dpkg --configure --pending

that will prompt to terminate any process currently holding these lock files, which, once killed will get the locks released. If you see an apt-get process or an aptitude process that looks stuck, killing them should be less harmful than when the packaging system is in the middle of a package installation. If the processes are really stuck and you have no other choice, you might need to kill them by passing -KILL instead of -TERM. You then need to finish any pending configuration so that dpkg can get those into a better state, and so that it can also integrate updates to its journal to the main status database.

Killing a dpkg process directly, if present, is in general not a great idea, because if dpkg is active, some maintainer script might be performing actions that are not resilient against abrupt termination (or crashes), but dpkg internally should be resilient to such abrupt terminations, and it's preferable to do that, than to remove any lock file, which has a way higher chance of damaging both the dpkg database and the filesystem.

Killing a frontend such as an apt-get or aptitude process, while not ideal, is in general much safer.

Remove your /var/lib/dpkg/lock file and force package reconfiguration.

sudo rm /var/lib/dpkg/lock
sudo dpkg --configure -a

It should work after this.

The most likely way to hit this is:

• boot Ubuntu
• start a terminal
• type sudo apt-get install whatever

and the command-line apt overlaps with update-manager automatically polling.

So if you try again in a few minutes that should fix it.

You will get this message if you forget to use sudo when executing an apt command.

Otherwise this is a sign that something else is installing or removing software and has locked the apt database while it performs the actions. The programs that can do this are:

• The Software Center
• The Update Manager
• The apt link installer (I think this now goes through SC)
• The apt-get or aptitude command line utilities.
• The Synaptic Package Manager

IMPORTANT: only try the below as a last resort since it can crash your system. First try killing any running instance of apt or aptitude as described in Faheem's answer.

You can force the lock off by removing the file, but it's not recommended without first closing the program that's holding the lock safely, since you could cause corruption or interrupt an installation (bad). The command provided by João should close the program that holds the lock and then remove the lock but won't protect you from install interruption:

sudo fuser -cuk /var/lib/dpkg/lock; sudo rm -f /var/lib/dpkg/lock   

And the same command can be used for the apt cache lock:

sudo fuser -cuk /var/cache/apt/archives/lock; sudo rm -f /var/cache/apt/archives/lock

Only one program can hold the lock. Make sure that you are not running aptitude, synaptic or adept. Close the program and run it again it should work.You may either have synaptic open, or have another terminal window open running apt-get, or have the update manager running.Check it and see if any of those are running,if any of them is running close it and try again.

Try this command in terminal to find what is running

ps -e | grep -e apt -e adept | grep -v grep

Note:
If that doesn’t print anything, type the following in terminal to remove the lock

sudo rm /var/lib/dpkg/lock    
sudo rm /var/cache/apt/archives/lock

Now you can install any Packages.

So far the best way to get it working without breaking a possible background running installation ( as it could happen by removing the lock file), is stopping the service using apt:

Error:

# sudo apt-get upgrade
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?`

Solution:

sudo systemctl stop apt-daily.timer

After upgrading the system I suggest re-enabling it, as the bug locking it could be fixed with the upgrade.

sudo systemctl start apt-daily.timer

I haven't verified this error gets fixed after upgrading. I'll add a new comment once I have that verified

First of all we should check what process created the lock file using lsof:

sudo lsof /var/lib/dpkg/lock

or in another situation where /var/lib/apt/lists/lock is problematic:

sudo lsof /var/lib/apt/lists/lock

The output will be close to something like:

COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF    NODE NAME
apt-get   12127 root   4uW  REG  252,1        0    86   /var/lib/apt/lists/lock

Then we should check what the commad is doing, we can find it out using ps, pgrep etc; the command is apt-get so I run:

pgrep apt-get -a

The -a switch lists the full command for me, in my case it's:

 pgrep -a apt-get
 12127 apt-get update

we can see that it's running update subcommand, I could run something like this too:

ps -f 12127

which produces:

UID        PID  PPID  C STIME TTY      STAT   TIME CMD
root     12127 12126  0 09:39 pts/0    S+     0:00 apt-get update

In this case I would wait for some minute for resource to be freed and if after 2 or 3 minute problem still exist or the command was something that I didn't care about or was not harmful for system (like this apt-get update) I send a SIGTERM to the process:

sudo kill -15 12127

It should do the work, If it didn't I'm going to send SIGINT this time (It's like pressing CTRL+C):

sudo kill -2 12127

If it didn't work too, we should send an SIGHUP (kill -1), and finally if nothing works I simply kill the process:

sudo kill -9 12127

or

sudo pkill -9 apt-get

Then I remove busy resources:

sudo rm /var/lib/apt/lists/lock

This error may be caused by the Update Manager trying to automatically refresh the list of packages in background, usually right after your login, thus locking the directory.

In this case just wait few seconds (or more, if your last update was long ago) for it to complete or launch Update Manager to check the status.

Don't be so fast to remove something, it may totally damage your system; rather wait until the currently installing or uninstalling program finishes its task and after that you will get access. If you think that there is nothing currently installing or uninstalling, then just reboot your system with the command sudo reboot.